Which HDD is not usable for WHM ? SDB (= md2)
What i must do to use the 2nd 1TB HDD to create accounts in WHM?
(cPanel create at the installation a /home folder at the HDD where all
accounts stored in)
I want to make the 2nd HDD usable as md2 with the /home2 folder on it.

Please help... i don't know which drive is in use and which one is free (currently not used).
Is md1 = SDA ???

At this time you are using only one HDD for your server. You have create only / paratition on your server that mean your cPanel and your hosting accounts are using that 1st HDD of your server. First You need to check your RAID and HDD configuration on your server.

Please provide me the following commands output so that we can assist you on this.